1. Field of the Invention
This invention relates to a laser source and process for producing coherent radiation at a blue wavelength, and more particularly relates to a modulated laser source and process for producing said radiation upconverted by nonlinear optical means with noncritical phase matching over a wide range of ambient temperature variation, rendering it especially suitable for optical storage applications.
2. Prior Art
Laser sources have long been regarded as the primary light sources for optical storage applications. For such applications, the laser source should be compact, reliable, low cost and have sufficient power for high data rate recording and reading of data on an optical storage medium. For writing data, rapid modulation capability of the laser source is advantageous. Also, the source should provide a short wavelength output to maximize storage density. It is known that semiconductor diode lasers, such as GaAlAs lasers, can be used to pump a compact, solid state laser such as a Nd:YAG laser. However, the wavelength of such diode pumped laser sources has been too long for optical storage applications or other applications requiring short wavelength (blue) coherent radiation.
It is known to use birefringent nonlinear crystals to mix laser beams of two different frequencies and hence differing wavelengths to produce an upconverted beam of a third frequency and correspondingly different wavelength or to generate from a fundamental input wave a harmonic wave by frequency doubling techniques. It is also known that such mixing processes require phase matching for optimum conversion efficiency.
If it is necessary to employ an input laser beam having a predetermined fixed frequency, it is known that the phase matching of the system may be adjusted by changing the angle at which the beam passes through the birefringent crystal. However, the angle is critical and must be controlled to very tight tolerance to achieve acceptable phase matching.
It is also known that if an input wave can be propagated essentially along any x, y or z crystallographic axis of a nonlinear crystal of orthorhombic or higher symmetry, noncritical (acceptable tolerance) phase matching is achieved. This desirably increases the adjustment angle tolerance to as much as about 10.degree.. However, this precludes adjustment by twisting the crystal. Also, it may require adjusting the ambient temperature to some unusual specific value and controlling that value within tight tolerance limits if operation is desired at some specific wavelength. Conversely, to operate at room temperature would require not only tight control of the temperature but operation at a corresponding critical wavelength.
The copending applcation of G. C. Bjorklund, U.S. Ser. No. 07/046,967, filed May 6, 1987, assigned to the assignee of the present invention, discloses a time modulated source of frequency doubled laser radiation in which a nonlinear crystal portion of a stripline electrical circuit is mounted within a laser cavity. The laser beam at a fundamental frequency impinges on and propagates through the nonlinear crystal. The stripline circuit is conditioned by a modulated high bandwidth electrical data signal to produce an output beam at a wavelength corresponding to the second harmonic of the fundamental frequency. The disclosed apparatus comprises a GaAlAs pumped Nd:YAG lasing rod, mirrors which are highly reflective at the fundamental wavelength, and a nonlinear crystal of potassium titanyl phosphate, KTiOP0.sub.4, (hereinafter KTP) or other suitable material. However, these nonlinear materials generally require critical phase matching.
There is a need for an improved time modulated source of laser radiation which exhibits noncritical phase matching over a wide range of operating temperatures, is capable of modulation at the high data rates required for viable optical storage applications, is controllable by voltages approaching TTL level voltages (of less than 5 volts), and which does not require application of an external electric field by a stripline circuit or other means.